A preferred form of projection lenses for wide screen television is disclosed in U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442, all assigned to the assignee of the present application.
In these previous patents, the lens units have been referred to as groups which perform specified optical functions. However, in accordance with present U.S. Patent and Trademark Office requirements, the overall lens will be defined in terms of "lens units". It will be understood that the term "units" refers to one or more optical elements or components air spaced from another optical unit.
It is well known that a specified optical function(s) of a lens unit or group in an overall lens may be accomplished by using one element or component or more than one element or component dependent upon the correction or function desired. A decision as to whether one or more elements is used as a lens unit in an overall lens design may be based on various considerations, including but not limited to, ultimate performance of the lens, ultimate costs of the lens, acceptable size of the lens, etc. Accordingly, in the following specification and appended claims, the term "lens unit" refers to one or more lens elements or lens components which provide a defined optical function or functions in the design of the overall lens.
The lenses disclosed in the aforementioned patents generally comprise three lens units: from the image end a first lens unit, having at least one aspheric surface, which serves as an aberration corrector; a second lens unit including a biconvex element which supplies all or substantially all of the positive power of the lens; and a third lens unit having a concave surface towards the image end of the lens, serving as a field flattener, and essentially correcting Petzval curvature of the first and/or second groups.
The lenses, as disclosed, are designed for use with a surface of a cathode ray tube (CRT). The lenses of U.S. Pat. No. 4,300,817, utilizing a single biconvex element in the second lens unit, all have an equivalent focal length (EFL) of one hundred twenty-seven millimeters or greater, while the lenses of U.S. Pat. No. 4,348,081, which utilize a two-element second lens unit, including the biconvex element, may have an EFL reduced to eighty-five millimeters as designed for direct projection for a five inch diagonal CRT. The lenses described in U.S. Pat. No. 4,526,442 are designed to have a fold in the optical axis between the first and second lens units and have been designed so that the EFL is as low as one hundred twenty-six millimeters. These EFL's are also for CRT screens having a viewing surface with an approximate five inch diagonal.
Projection TV sets are rather bulky and have required high volume cabinets. One manner of reducing the cabinet size is to decrease the EFL of the projection lenses.
The EFL of the lens is a function of the total conjugate distance between the CRT and the display screen. This is shown by the relationship:
OL=EFL(1+1/M)+EFL(1+M)
where
OL is the overall conjugate distance of the system from object to image PA1 EFL (1+M) is the distance from the image to the first principal point of the lens PA1 EFL (1+1/M) is the distance from the object to the second principal point of the lens PA1 M is the magnification of the system expressed as the ratio of object height to image height.
and
Therefore, in order to decrease the total distance between the CRT and the screen, it is necessary to reduce the EFL. This, of course, increases the field angle of the lens, if that lens is to be used at the same magnification.
An effect of increasing the angular coverage of the lens as a result of decreasing the EFL is that the aberrations become more difficult to correct. A single biconvex element second lens unit, as shown in the aforementioned patents, does not provide the lens designer adequate degrees of freedom to correct for the resulting astigmatism and distortion. By dividing the optical power of the second lens unit, as disclosed in U.S. Pat. No. 4,348,081, a better control of aberrations can be obtained for a shorter EFL. However, merely splitting the optical power of the second lens unit into two elements to obtain additional degrees of optical design freedom does not provide acceptable contrast and resolution where the angular coverage of the projection lenses is required to be in excess of twenty-seven degrees, semi-field.
A further consideration is introduced wherein a spacing is provided between the phosphor screen of the CRT and the third lens unit of the projection lens. This spacing may be required for the inclusion of a liquid cooling and/or optical coupling material and a housing necessary to enclose the coolant against the face of the CRT. This additional spacing between the face of the CRT causes the third negative lens unit to contribute more negative power, which must be compensated by increased power in the positive second lens unit.
Projection lens of the overall type described have been designed with decreased EFL's by designing a more complex second lens unit split into more than one lens element, as exemplified in the lenses disclosed in co-pending Applications Ser. Nos. 642,825 and 652,062, issued Aug. 21, 1984 and Sept. 19, 1984, respectively.
These designs are currently used on many wide screen projection television sets and may have an equivalent focal length as low as eighty millimeters.
Co-pending application Ser. No. 776,140, filed Sept. 13, 1985, discloses projection lenses in which the EFL is reduced to less than sixty millimeters for an object height of approximately five inches.
These lenses use a negative first lens unit to aid in correction of field curvature due to the large positive power of the second lens unit and increased field angle. This approach works very well and leads to high quality optical performance of the lens. However, it requires large diameter positive elements in the second lens unit to accommodate the diverging bundle of light (as traced from the long conjugate). This construction also requires a lens of relatively long front vertex distance (FVD) largely due to a long space between the first negative element and the following power unit necessary to achieve an appropriate correction of the field curvature and astigmatism. The front vertex distance is the distance from the image side of the first lens unit to the face place of the CRT.
The related co-pending application Ser. No. 820,553 discloses a lens of the type described, which consists of a first lens unit which comprises a single element with two aspheric surfaces, and an overall positive meniscus shape preferably convex to the image end, a second positive element, and a third lens unit having a strongly negative surface concave to the image end. The first lens element is of positive optical power at the optical axis of the lens, but due to the aspheric power of the surfaces, the positive optical power decreases with distance from the optical axis until it become strongly negative at or closely adjacent the clear aperture of the first lens element, as hereinafter explained.
The strong negative power of the third lens unit contributes to correction of the Petzval sum of the other lens elements. The strongly concave surface may be made aspheric to also correct for residual astigmatism and field curvature of the lens. The second lens element provides the majority of the positive power of the lens and some correction for astigmatism. The first lens element must then correct the aperture dependent aberrations, particularly, spherical and coma. Lenses as described in the related co-pending application are very compact, having the first and second lens units spaced more closely than heretofore. Lenses as described may have a field angle as great as 73.degree. while comprising only three elements.
In lenses described in U.S. Pat. No. 4,300,817 and the co-pending applications, the power unit elements are often made out of acrylic because of simplicity of manufacturing aspherical surfaces on plastic. However, the refractive index of acrylic varies significantly with the change of temperature. As a result, the focal length of the lens where all the elements are made out of acrylic may vary quite substantially with the change of operating temperature leading to a change of focus and to the degradation of image quality. One way to compensate for the focus shift with temperature is to design a lens mount and a lens barrel using, possibly, a bi-metallic plate or other means that will shift the position of the lens relative to CRT as a function of temperature in such a way that the focus of the lens will remain in a constant position.
Another way of solving the problem of thermal focus shift is to make the power group out of glass because the index of refraction of glass is much more stable with the change of temperature. Consequently, the power of the lens will not be changing by very much and the same will be true for a position of the focus of the lens. However, since making aspherical surfaces on glass is currently much more expensive than on plastic, the reasons of economics limit this glass power unit to having only spherical surfaces. With this limitation, it becomes very difficult to obtain a good optical performance of the lens while, at the same time, maintaining a simple configuration of that lens and still be capable of handling high speed and wide field coverage.
The present invention provides a projection lens for projecting a monochromatic CRT onto a larger screen, the lens capable of handling a high speed of at least f/1.0; providing a wide field of view in excess of 30 degrees and up to 45 degrees; exhibiting very little focus shift and image quality degradation with the change of operating temperature at which this lens is used; being manufactured very economically and in large numbers using conventional technology; and delivering a high image quality.